1. Field of the Invention
The invention relates to a method of making measurements on and between existing objects located underwater. More particularly, the invention relates to controlling the drift of an inertial navigation system using a range and bearing image capture system such as a laser camera or other sensors to make measurements without the system being in contact with an underwater object or the seabed. The spatial relationship between the inertial navigation system and the underwater object is achieved by range and bearing measurements.
2. Description of the Related Art
Any motion of an object in space is composed at the microscopic level of two basic motions: micro-rotations and micro-linear motions. An inertial navigation system (hereinafter “INS”) measures these motions using gyroscopes (to sense angular motions) and accelerometers (to sense linear motions). The INS may measure rotations using the gyroscope and linear motions using the accelerometers at a high frequency (typically 500 times per second). Even given this high sample rate, the INS is subject to error. Because of the imperfection of the accelerometers and gyroscopes, INS navigation, when not controlled, is subject to a continuous drift of about 0.8 miles per hour in a more accurate INS, and several miles per hour in one that is less accurate.
This drift is especially problematic when the INS is used in underwater metrology i.e., the science of measuring accurately the position and orientation of objects and physical structures underwater (typically on the seabed). The most common metrology consists in measuring the distance between the two flanges of a pipeline system that will receive the ends of a jumper or spool, i.e. the hard, prefabricated connecting pipes that join a pipeline system (typically ending at a manifold) and a wellhead. The accurate measurement of distance between receptacles of the pipeline and the wellhead, to which each end of the jumper is to be attached, is crucial to ensure accurate manufacturing of the jumper. To that end, the exact distance between the pipeline receptacle and the wellhead receptacle, the difference in depth between them and the exact 3D orientation (heading, pitch and roll) of each receptacle must be accurately measured.
To correct for drift in underwater metrology, the INS navigation is sometimes corrected using an “aiding by zero velocity update”, or ZUPT method. The ZUPT is a period without motion where the INS is parked on some structure on the seabed: usually either one of the two receptacles that need to be measured. When the INS is parked for at least 15 seconds, the INS recalibrates and corrects any navigation drift from the knowledge that, when there is no motion the readings of the accelerometers and gyroscopes must be caused by noise (pure erroneous measurements). The drawback of such methods is that these require that the INS be carried by a robot (such as a Remotely Operated Vehicle or “ROV”), other vehicles, or a diver, to touch the flange or receptacle (actually: the ROV rests on the receptacle for at least 15 seconds), potentially causing damage. Even more, in order to measure accurately the position and orientation of those flanges, a mating system (stabbing guide) is often specially made and installed on or near the two flanges according to these traditional metrologies which can be extremely cumbersome.